The subject matter of the present invention pertains to means for preventing compromising emanations from electronic equipment; in particular, electronic equipment employing a cathode-ray tube as an information display element.
As defined by the U.S. Air Force in its publication entitled What You Should Know About Compromising Emanations (Tempest), OPR: AF COMSEC Center, a compromising emanation is "electromagnetic energy unintentionally emitted from equipment processing classified information and having some characteristic that makes it possible to intercept and recover the classified information." In the military sense, for a device to emit a compromising signal, the device must contain classified information. If there is no classified information, there can be no compromising signal. In the more general sense, a compromising emanation may be considered to include any detectable signal containing information, for example, industrial information, that a device user would rather not have known outside his or her organization.
Several schemes have been developed for reducing or eliminating unwanted emanations from electronic equipment. The most common scheme is to enclose the equipment completely within a surrounding metal enclosure connected electrically to ground or other common level of electrical potential. A special case arises, however, when the equipment being treated includes a visual display element, such as a cathode-ray tube, the display screen of which cannot be hidden behind a solid metal barrier.
Known prior art schemes for shielding cathode-ray tube display screens against unwanted non-visual emanations include a woven metallic screen soldered or welded to a surrounding metal frame placed in front of the display screen and connected to a metal enclosure surrounding the remainder of the tube. An example of such a shield is disclosed in Lill et al U.S. Pat. No. 3,952,152 and, according to Lill et al, in Simms et al U.S. Pat. No. 2,673,342.
A more pertinent scheme comprises the lamination of a fine wire mesh between two panels of glass to form an essentially transparent shield that may be placed over the display screen of a cathode-ray tube and its wire mesh connected electrically to a metal enclosure surrounding the remainder of the instrument. To facilitate connecting the wire mesh to the metal enclosure, the mesh is folded around the edge of the glass panels to form a peripheral border which may be pressed into contacting engagement with a similarly configured frame that provides the actual connection to the enclosure. A principal disadvantage of such a scheme is that it requires the frame to conform in size and shape to the glass panels. This disadvantage is particularly evident when the shield is curved. In such a case, the metal frame must also be curved; a requirement calling usually for a frame of rather expensive molded or milled construction.